Abstract

This ZIP describes a proposed specification for a standardized format for clients who wish to transmit or receive content within the encrypted memo field of shielded transactions.

Motivation

A well-defined standard for formatting content within the encrypted memo field will help expand its use within the Zcash ecosystem by providing a commonly recognized format for memo values carrying different types of data. Users and third-party services benefit from standardized formatting rules that define the type and length of the data contained within.

Specification

Section 5.5 of the Zcash protocol specification 1 defines three cases for the encoding of a memo field:

• a UTF-8 human-readable string 2, padded by appending zero bytes; or
• the byte 0xF6 followed by 511 0x00 bytes, indicating "no memo"; or
• any other sequence of 512 bytes starting with a byte value 0xF5 or greater (which is therefore not a valid UTF-8 string), as specified in ZIP 302.

This ZIP refines the specification of the third case.

The following specification constrains a party, called the "reader", that interprets the contents of a memo. It does not define consensus requirements.

• If the first byte (byte 0) has a value of 0xF4 or smaller, then the reader MUST:
  • strip any trailing zero bytes
  • decode it as a UTF-8 string (if decoding fails then report an error).
• If the first byte has a value of 0xF6, and the remaining 511 bytes are 0x00, then the user supplied no memo, and the encrypted memo field is to be treated as empty.
• If the memo matches any of these patterns, then this memo is from the future, because these ranges are reserved for future updates to this specification:
  • The first byte has a value of 0xF6, and the remaining 511 bytes are not all 0x00.
  • The first byte has a value between 0xF7 and 0xFE inclusive.
• If the first byte has a value of 0xFF then the reader should not make any other assumption about the memo. In order to put arbitrary data into a memo field (that might have some private non-standard structure), the value of the first byte SHOULD be set to 0xFF; the remaining 511 bytes are then unconstrained.
• If the first byte has a value of 0xF5, then the reader should not make any other assumption about the memo. This value was used ambiguously in the past by private agreement; applications SHOULD prefer 0xFF which is unambiguously for this purpose.

Rationale

The new protocol specification is an improvement over the current memo field content specification that was in the protocol spec up to version 2020.1.0, which stated:

The usage of the memo field is by agreement between the sender and recipient of the note. The memo field SHOULD be encoded either as:

• a UTF-8 human-readable string [Unicode], padded by appending zero bytes; or
• an arbitrary sequence of 512 bytes starting with a byte value of 0xF5 or greater, which is therefore not a valid UTF-8 string.

In the former case, wallet software is expected to strip any trailing zero bytes and then display the resulting UTF-8 string to the recipient user, where applicable. Incorrect UTF-8-encoded byte sequences should be displayed as replacement characters (U+FFFD).

In the latter case, the contents of the memo field SHOULD NOT be displayed. A start byte of 0xF5 is reserved for use by automated software by private agreement. A start byte of 0xF6 or greater is reserved for use in future Zcash protocol extensions.

See issue #1849 for further discussion.

Backwards Compatibility

Encrypted memo field contents sent without the standardized format proposed here will be interpreted according to the specification set out in older versions of the protocol spec.

References